A major pest of stored tobacco and tobacco products is the cigarette beetle, Laisoderma serricorne. During the past 50 years, toxic fumigants such as hydrogen cyanide, methyl bromide, and hydrogen phosphide have been used to fumigate tobacco and other agricultural products for control of the cigarette beetle and other stored product insects. Usage of these and other fumigants has become increasingly restricted during the past several years because of regulatory agencies' concern with worker exposure to pesticides, pesticide residue on agricultural products, fumigant flammability, and contamination of air and water. As a result of these and other regulatory actions, hydrogen phosphide has become the fumigant of choice for disinfestation of stored agricultural commodities and products. Hydrogen phosphide (PH.sub.3) is the toxic gas released when formulations of aluminum or magnesium phosphide are exposed to air. Although phosphide fumigants are considered relatively safe to use, they are classified for restricted use only by the Environmental Protection Agency (EPA). Therefore they must be applied only under the supervision of certified or licensed applicators. Usage of phosphide fumigants has become more restricted and controlled during the past three years as a result of emergency planning and reporting requirements that were specified in Title III of the Superfund Amendments and Reauthorization Act (SARA) of 1986, and labeling changes made by the 1987 EPA Amended Registration Standard for Phosphide Fumigants. Moreover, states and localities are now beginning to establish regulatory limits on amounts of hydrogen phosphide and other chemicals that are emitted into the atmosphere. When used as a fumigant, hydrogen phosphide is released into the atmosphere after the fumigation period has ended, i.e., during the period of warehouse aeration.
An alternative to toxic fumigants for disinfestation of agricultural commodities and products is asphyxiation of insects by means of low oxygen environments. This derives from the ancient practice of hermetic storage where grains, beans, etc., were stored for extended periods in sealed underground pits. In such an environment, the respiration of both the commodity and insects present in the commodity depletes oxygen to a level of approximately 2 percent, and increase carbon dioxide content to about 13 percent.
Although hermetic storage is still practiced in a few mid-eastern countries, it is now considered impractical as a modern means of insect control for agricultural products. The use of modified atmosphere by other means, however, is effective and practical for control of stored product insects. In a modified atmosphere, disinfestation occurs much faster than with hermetic storage; but like hermetic storage, there is no chemical residue left to remain in the product. Modified atmosphere occurs when the existing atmosphere within a storage or container is purged and replaced with carbon dioxide, nitrogen, or combustion gases.
Carbon dioxide is readily available in most localities as a compressed liquid or solid (dry ice). CO.sub.2 is considered generally safe for workers in concentrations up to the Threshold Limit Value (TLV) of 10,000 ppm, compared to 0.3 ppm for hydrogen phosphide. It is also nonflammable, exempt from residue tolerance limits required by EPA for most raw and processed agricultural products, and aerates rapidly from treated commodities.
In recent years, numerous investigators have reported on the advantages of carbon dioxide for protection of agricultural products against insects. See Childs, D. P., Overby, J. E., "Mortality of the Cigarette Beetle in High-Carbon Dioxide Atmosphere," J. Econ. Entomology 76:554-546, 1983; Harein, P. K., Press, A. F. Jr., "Mortality of Stored-Peanut Insects Exposed to Mixtures of Atmospheric Gases at Various Temperatures," J. Stored Prod. Res. 4: 77-82, 1968; Jay, E. G., Pearman, G. C. Jr., "Carbon Dioxide for Control of An Insect Infestation in Stored Corn (Maize)," J. Stored Prod. Res. 9: 25-29, 1973; Keever, D. W., "Use of Carbon Dioxide to Disinfect a Tobacco Warehouse of Cigarette Beetles," J. of Agri Entomology 6: 43-51, 1989; and USDA, "Methods of Applying Carbon Dioxide for Insect Control in Stored Grain," USDA, Sci. and Ec. Asms, Advances in Agri Technology, AAt-S-13/April 1980. These reports on laboratory and field experiments have shown that CO.sub.2 is an effective disinfestant for control of the cigarette beetle and other stored product insects. The basic method of treatment used by these investigators is as follows: Products to be disinfested are placed into a closed container or chamber. The container is equipped with a purge or exhaust port and a port for admitting CO.sub.2. As pressurized CO.sub.2 is injected into the container, the ambient air is evacuated through the exhaust port. When the concentration of CO.sub.2 reaches the desired level within the container, the exhaust port is closed and CO.sub.2 supply is turned off or reduced to a lower flow rate. During the period of disinfestation, CO.sub.2 concentration is monitored periodically (i.e., on 12 or 24 hr cycles). When the concentration decreases below the desired level, additional "make-up" CO.sub.2 is added by manual operation of switches and valves. For monitoring CO.sub.2 levels, air from inside the chamber is aspirated through a gas sampling line (i.e., polyethylene tubing) by manual means utilizing a hand held bellows, a piston-type pump, or a battery-operated suction pump. A color-changing detector tube, through which a calibrated volume of air is drawn by the pump, indicates the CO.sub.2 concentration.
The time required for complete disinfestation of a container or chamber varies from a few days to approximately two weeks. Variables affecting this time requirement are CO.sub.2 concentration within the chamber and temperature of the tobacco or tobacco products. The quantity of CO.sub.2 required is also dependent on product temperature, CO.sub.2 concentration, and how well the chamber is sealed against leaks. In colder weather, additional heat is usually required to warm the product in order to increase the respiration rate of insects. The rate of insect kill is directly proportional to both CO.sub.2 concentration and temperature.
During the period of disinfestation, especially the first 24-48 hours, CO.sub.2 concentration decreases from the initial level. The decrease occurs as a result of leakage from the chamber and absorbance of CO.sub.2 by the product. In order to minimize such fluctuations in CO.sub.2 concentration, it is necessary to make periodic measurements on-site, followed by manual adjustments to supply additional CO.sub.2. If close control of CO.sub.2 concentration is desired, round-the-clock attention is required. If close control is sacrificed, excessive amounts of CO.sub.2 may be used in order to ensure that the minimum concentration is maintained.
For these reasons, proposals to date for CO.sub.2 use are generally impractical for any large scale commercial applications. It would be desirable to provide an apparatus for treating large quantities of agricultural products, which is automatic in operation, and which ensures a proper application of CO.sub.2 or other treatment gas. It would also be desirable to provide a system which treats products contained in mobile storage containers, and which can carry out a disinfestation process independently on multiple containers.